1. Field of the Invention
The present invention relates to a metal honeycomb assembly, particularly one useful as a catalyst carrier for exhaust gas purification.
2. Description of the Related Art
It is a recent trend that a catalyst carrier inserted in the exhaust gas path of an automobile is made of metal instead of ceramic. A metal catalyst carrier is advantageous because of a better heating property and a reduced pressure loss in comparison with a ceramic catalyst carrier.
Japanese Unexamined Patent Publication (Kokai) No. 55-87815 discloses a metal catalyst carrier in the form of a honeycomb assembly as shown in FIG. 1, which comprises a metal honeycomb column 1 composed of a flat metal sheet 2 and a wavy or corrugated metal sheet 3 laminated and spirally wound together; and a metal case 4 fittingly enclosing the metal honeycomb column 1.
Laser welding is often recently used to bond the flat and wavy sheets 2 and 3, and also to bond the outermost circumferential portion of the wavy sheet 3 to the metal case 4. Laser welding has a higher productivity than conventional brazing or soldering which requires high-temperature treatment in an inert-gas atmosphere.
However, the following problem occurs when the layer of wavy metal sheet 3 that forms the outermost portion of the honeycomb column 1 is laser-welded to the metal case 4.
The flat metal sheet 2 and the wavy metal sheet 3 are in the form of a foil as thin as about 50 .mu.m, for example, to reduce the pressure loss and for other purposes. On the other hand, the metal case 4 has a wall thickness of several millimeters, usually about 1.5 mm, to provide the necessary strength to the catalyst carrier.
Because of this difference in thickness, a laser beam applied with a sufficient energy for penetrating the thick wall of the case 4 occasionally opens a hole in the thin wavy sheet 3, or a laser beam applied with reduced energy to prevent the formation of such a hole causes insufficient fusion, with the result that both cases can cause a poor weld-bond strength. Furthermore, premature failure occasionally occurs in the portion near the bond between the thin wavy sheet 3 and the case 4.